This invention relates to boost switching power converters.
Boost switching power converters accept power from an input voltage source and deliver power to a load at a controllable load voltage value which is greater than the voltage delivered by the input source. Such converters are useful in applications where a load must be supplied with a voltage greater in magnitude than the available source voltage, or where the magnitude of the voltage delivered by an input source may, under either steady-state or transient conditions, drop below the minimum value of operating voltage required by the load. In one increasingly important application, a boost switching converter forms the core element of a power factor correcting AC to DC preregulator. In such preregulators, an AC voltage source is rectified and delivered to the input of a boost switching power converter. The boost switching power converter is controlled so as to maintain the load voltage at or above both the peak value of the AC source voltage and the minimum operating voltage of the load, while simultaneously forcing the boost converter input current to follow the time varying periodic waveform of the AC source. In this way, the voltage delivered by the boost switching converter is controlled to be within the operating voltage range of the load, while the power factor presented to the AC source is kept at essentially unity. Examples of preregulators of this type are described in Wilkerson, U.S. Pat. No. 4,677,366, Williams, U.S. Pat. No. 4,940,929, and Vinciarelli, U.S. patent application Ser. No. 07/642,232, filed Jan. 16, 1991. One such prior art boost switching converter is shown in FIG. 1. In the Figure, an input inductor 12 is connected in series with an input voltage source 14, of magnitude Vin, and a switch 16. A diode 18, connected between the junction of the input inductor and the switch, is poled to carry current towards an output capacitor 20 and a load 22. In operation, the frequency at which the switch 16 is turned on and off during a converter operating cycle is fixed, and the duty cycle of the switch (i.e., the fraction, D, of the time that the switch is on during an operating cycle) is varied as a means of controlling the converter output voltage, Vo. The inductor smooths the input current, I1, keeping it essentially constant throughout the operating cycle, and the output capacitor smooths the effect of variations in the current Io so that the converter delivers an essentially DC output voltage. When the switch is on, the voltage across the switch is zero (assuming ideal circuit elements) and all of the input current flows losslessly in the switch; when the switch is off all of the input current flows through the diode toward the capacitor and the load and the voltage across the switch is equal to the output voltage Vo. Under steady state conditions, the average voltage across the input inductor must be zero, else the average value, Iin, of the input current, I1, will vary. Thus, the average value of the voltage across the switch, (1-D).multidot.Vo, must equal Vin, hence Vo=Vin/(1-D). Since D must be between zero and one, Vo&gt;Vin. In prior art boost switching converters of the kind illustrated in FIG. 1, neither the switch nor the diode is ideal, and both elements contribute to converter losses. When the switch is turned on it is exposed to both the current flowing in the input inductor and to a reverse current which flows from the output capacitor back through the diode during the diode reverse recovery time. Switch turn-off occurs when the switch is carrying the full converter input current. Since both the rise and fall time of the switch are finite, the presence of switch voltage and current during the switch transition times will cause power to be dissipated in the switch, and, all other conditions being equal, these switching losses will increase directly with converter operating frequency. Thus, although increased operating frequency is desirable in that it allows reducing the size of the input inductor and the output capacitor (and hence the size of the converter), prior art converters inherently must trade power density against operating efficiency. As a practical matter, as the operating frequency of a prior art boost switching power converter is raised much beyond 100 KHz, efficiency declines rapidly and the thermal and electrical stresses on the switch become unmanageable. Another characteristic of prior art boost switching converters is that two or more units connected to a common input source and load will not inherently share load power if operated synchronously. Current sharing between units connected in this way is first-order dependent on second-order effects (e.g., diode voltage drops, switch impedance).